Band edge  emission enhanced organic light emitting diode utilizing chiral liquid crystalline emitter

ABSTRACT

Disclosed herein is a light emitting device and method of manufacturing such a device comprised of a series of photopolymerizable, chiral liquid crystalline layers that can be solvent cast on a substrate. The mixture of chiral materials in each successive layer may be blended in such a way that each layer has the same chiral pitch. Further the chiral materials in each layer may also be blended so that the ordinary and extraordinary refractive indices in each layer match the other layers such that the complete assembly of layers will optically function as a single relatively thick layer of chiral liquid crystal. The chiral nematic material in each layer can spontaneously adopt a helical structure with a helical pitch. The light emitting layers of the light emitting device can further comprise electroluminescent material that emits light into the band edge light propagation modes of the photonic crystal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims the benefit under 35U.S.C. § 120 of the earlier filing date of, copending U.S. Ser. No.15/999,863 filed on Aug. 22, 2018, which is a 371 National Stage ofInternational Application No. PCT/US2017/21867, filed Mar. 10, 2017,which was published as International Publication No. WO 2017/156433, andwhich claims the benefit under 35 U.S.C. § 119(e) of the earlier filingdate of U.S. Provisional Patent Application No. 62/306,128 filed on Mar.10, 2016, the disclosure of which is incorporated by reference herein.

BACKGROUND

U.S. Provisional Application 62/183,771 filed on Jun. 24, 2015 is herebyincorporated by reference in its entirety.

In U.S. Pat. No. 7,335,921 filed May 8, 2003, U.S. patent applicationSer. No. 10/434,941 filed May 8, 2003, and U.S. Pat. No. 9,129,552 filedMay 8, 2003 light emitting diode devices (LEDs) and particularly organiclight emitting diode devices (OLEDs) are described in which one or morephotonic crystal structures are integrated with the light emitting diodestructures so as to provide enhanced levels of light emission and energyefficiency through the phenomenon of stimulated emission. The devicesdescribed include both laser and non-laser devices. The principle ofoperation of these devices, that have been collectively termed feedbackenhanced organic light emitting diodes (FE-OLEDs), is that light is fedback into the light emitting layers of the devices from the photoniccrystals, or if only one photonic crystal from other reflectivestructures so as to yield stimulated emission of light from the lightemitting layers. Since the direction of propagation of the feedbacklight is vertical to the plane of these devices, and since lightproduced by stimulated emission propagates in the same direction as thelight stimulating the emission, nearly all light emitted from FE-OLEDsis emitted in the direction perpendicular to the plane of the device.That is to say, the devices are vertically emitting.

The nearly complete vertical emission of light in the FE-OLEDs resultsin greatly increased efficiency of light generation versus power inputbecause light is not emitted with a substantially in-plane direction ofpropagation. Light that is emitted with a substantially in-planedirection of propagation in OLEDs and LEDs is trapped within the devicesby reflection from layer to layer interfaces within the devices andeventually absorbed in the devices creating heat. Thus the verticalemission of light in FE-OLEDs results in improved energy efficiency.

FE-OLEDs are generally described as either Type 1 or Type 2. The first(Type 1 device) can be characterized as operating by means of lightgeneration within a defect in a one-dimensional photonic crystal.Photonic crystals are dielectric media that have a periodic variation ofrefractive index of light through their extent. A result of the periodicvariation of refractive index is that the wave equation for lightpropagation has no solutions over a range of wavelengths (the stop band)for light propagating in the direction(s) of periodic index variation. Aresult is that a light emitting material molecule embedded in thephotonic crystal medium cannot emit light having wavelengths within thestop band in the direction(s) of periodic index variation. A secondresult is that light propagating external to the photonic crystal mediumwill be completely reflected from the medium's surface if it has awavelength within the stop band and if it is traveling in the directionor in one of the directions of refractive index variation. The photoniccrystal structures utilized in FE-OLEDs are usually one-dimensional innature with the axis of refractive index variation perpendicular to theplanes of the devices, in other words it is parallel to the transmissionaxis of the device, but higher dimensional photonic crystals may also beused.

In a Type 1 device there is a planar region (a defect) constituting acavity within the photonic crystal medium and parallel to the plane ofthe device in which the regular cyclic variation of refractive indexdoes not occur. Another way of describing this structure is that thereis a phase-slip in the photonic crystal medium that produces a defect.If a light emitting molecule is embedded within the defect, the lightthat it emits within the wavelength stop band will be trapped within thedefect by the reflective properties of the photonic crystal medium. Thethickness of the defect can be made quite small with the result that thephoton density of light trapped in the defect can become quite high.This results in very efficient stimulation of light emission fromexcited state molecules embedded in the material in the defect.

Viewed classically the defect in a Type 1 device is a micro-cavitybetween two photonic crystal reflectors. Light produced by emittermolecules in the micro-cavity is reflected back and forth between thetwo reflectors stimulating more light emission.

While the Type 1 devices take advantage of the light reflectionproperties of photonic crystal structures, Type 2 devices depend on theproperties of light emitted inside photonic crystals. As describedabove, a stop band is created inside a photonic crystal structure. Thisis a range of wavelengths over which there are no propagation of lightallowed through the cyclically varying refractive index structure. Itwould be tempting to think that the light propagation modes or statesthat exist in the stop band wavelengths in free space are destroyed whenthe photonic crystal structure is introduced around the light emitter.However, this is not the case. What actually happens is that the lightpropagation states are expelled from the stop band and are “stacked up”at its edges in the wavelength spectrum. This sort of behavior isgenerally quantified in terms of the density of states, that is to saythe number of allowable wave propagation states or modes per interval ofenergy in the electromagnetic spectrum. Plots of the density of statesversus light frequency for free space (dashed line) and for a photoniccrystal medium (solid line) are shown in FIG. 1. In the stop bandwavelength region between frequencies B and C, the density of states iszero. However, at wavelengths A and D that are slightly red and blueshifted respectively from the boundaries of the stop band, this“stacking up” phenomenon means that the densities of states in thephotonic crystal medium are considerably higher than for lightpropagating in free space. Since the amount of light that an emittingmolecule will emit into its surrounding depends on the density of statesavailable to propagate the light, a light emitter in the photoniccrystal of FIG. 1 will emit considerably more light at frequencies A andD than it would in free space.

A further property of photonic crystals is that at wavelengths (orfrequencies) close to the stop band, the medium is partiallytransmissive and partially reflective. The result is that light emittedat these wavelengths builds up within the medium as it is localizedabout the point of emission. The combination of the higher than normallevel of photon emission at wavelengths adjacent to the stop bandcombined with buildup of these photons within the medium due to internalreflections yields very high photon densities throughout the bulk of thephotonic crystal medium. These high photon densities in turn ensurestimulated emission from essentially all the excited state emittermolecules embedded in the photonic crystal. Because of theone-dimensional nature of the photonic crystal structure used, thepropagation direction of the light produced by stimulated emission isall in the vertical direction, just as it was in the Type 1 devices, theenergy efficiency of the Type 2 devices is similarly enhanced.

Laser devices have been demonstrated in the form of chiral lasersutilizing a chiral liquid crystal doped with a fluorescent dye. Thestructure of an aligned chiral liquid crystal is shown in FIG. 2. Rodshaped liquid crystal molecules 210 within layers 220 tend to align withtheir long axes pointing in single direction parallel to an axis calledthe director 230. In aligned nematic liquid crystals the director alwayspoints in the same direction. However, because of the asymmetic shape ofsome of the molecular constituents, in chiral liquid crystals as onepasses down through the material along axis 240 as shown in FIG. 2, thedirection of the director 230 rotates so as to sweep out a helix. Thusthe individual liquid crystal molecules combine into a helicalstructure. Because of the anisotropy of electronic polarization withinthe individual molecules, light of the proper circular polarizationpassing through the liquid crystalline material encounters a much higherrefractive index when its associated electric vector is parallel to themolecules' long axes than when its electric vector is perpendicular tothe molecules long axes. Given that the light has the proper wavelength,light of the opposite circular polarization (for instance, right-handedversus left-handed) has its associated electric vector rotated so as totrack the helical structure of the liquid crystal and thus sees nochange in refractive index as it passes up through the chiral liquidcrystalline structure. Thus circularly polarized light of the correcthandedness propagating parallel to axis 240 sees a periodicallyoscillating refractive index as it passes through molecules with theirlong axes oscillating between directions parallel and anti-parallel tothe light's electric vector. In this way the assembly of chiral liquidcrystalline molecules act as a one-dimensional photonic crystal materialfor one circular polarization of light along axis 240. The medium willhave a stop band for light emission similar to that shown in FIG. 1 andthere will be enhancement of the density of states near the stop bandedges in the electromagnetic spectrum. If the fluorescent dye doped intothis aligned chiral liquid crystal is excited to emit light (forinstance, by optical pumping with light from a YAG laser) and the lightproduced has in its spectral band a wavelength in the chiral medium thesame as one of the wavelengths at the stop band edges (analogous to Aand D in FIG. 1) a buildup of photon density will occur at that bandedge wavelength. These photons will stimulate further emission. In thechiral laser there is more than sufficient stimulated emission andlittle enough absorptive losses to enable lasing. These optically pumpeddye doped liquid crystal lasers operate on a similar principle as a Type2 solid state device except that light emission is excited by opticalexcitation rather than electrical excitation as in a light emittingdiode. Another important aspect of the chiral liquid crystal laserdevices is that the fluorescent emitter material was embedded throughoutthe chiral liquid crystal medium. This yielded a wide enoughdistribution of fluorescent material to produce sufficient laser gain toinitiate lasing.

Producing a solid state Type 2 device encounters immediate issuesstemming from the structures of conventional LEDs and organic lightemitting diodes (OLEDs) an example 300 of which is portrayed in FIG. 3.The OLED 300 consists of a substrate 310, a transparent anode 320, ahole injection layer 330, a hole transporting layer 340, an emitterlayer 350, an electron transporting layer 360, and a metal cathode 370.The device 300 functions as follows: when an electrical potentialdifference is applied between the anode and the cathode, positivelycharged holes are injected from the anode 320 into the hole injectionlayer 330. Under the influence of the imposed electric field the holesflow from the hole injection layer, through the hole transporting layer340 and into the emitter layer 350. At the same time electrons areinjected from the cathode 370 into the electron transporting layer 360.Under the influence of the imposed electric field the electrons flowfrom the electron transporting layer into the emitter layer 350. In theemitter layer the electrons and holes pair together on single organicmolecules promoting the molecules into electronically excited states.These excited states (excitons) then collapse to produce light.

In OLEDs the hole injection layers 330, the hole transporting layers340, the emitter layers 350, and the electron transporting layers 360are all composed of organic materials, which have quite low chargecarrier (electron or hole) mobilities as compared to typical inorganicsemiconductor materials. In current OLEDs the total thickness of all theorganic layers is less than 200 nanometers and the emitter layers havethicknesses in the range of 25 to 50 nanometers.

Inorganic Type 1 devices in which the emitter layer is located in adefect or micro-cavity between two photonic crystal structures aredescribed in U.S. Pat. No. 4,949,350 which teaches Type 1 devices inwhich the light emitting material is an inorganic semiconductor such asIn_(0.2)Ga_(0.8)As. An example of the structure of these devices isportrayed in FIG. 4 and FIG. 5. The photonic crystal structures aboveand below the cavity containing an emitter consist of epitaxially grownalternating layers of AlAs (lower refractive index) and GaAs (higherrefractive index) with layer thicknesses corresponding to one-quarterwave thickness for light with the wavelength of 900+nm. wavelengthdesired from the device.

FIG. 4 presents a top level view of the device 400. The device consistsof an transparent electron conducting substrate 410, an electronconducting photonic crystal mirror 420, a spacer layer 430 that performsmuch the same function as an electron transporting layer in OLEDs, anemitter layer 440, a second spacer layer 450 that performs much the samefunction as a hole transporting layer, a hole conducting photoniccrystal layer 460, a gold anode, and electrical contacts 480 and 490.

FIG. 5 presents a more detailed view of layers 420 and 430. The electronconducting photonic crystal mirror 420 consists of twenty pairs 510 ofan aluminum arsenide 520 and a gallium arsenide 540 layer. Between eachlayer pair is a “superlattice” 530 consisting of four extremely thin(1.7 nanometers) alternating GaAS and AlAs layers. These superlatticesare included to improve electrical conductivity and have no opticalfunction. Also shown in FIG. 5 are the sub-layers that compose the firstspacer layer 430. These are an ALAs layer 550, another superlattice 560,and a GaAS layer 570. Also shown is a graded index layer 580 that isintended to further confine light into the emitter layer. Photoniccrystal layer 460 is also built up from alternating AlAS and GaAs layersjust as photonic crystal layer 420 was. In this example twelve layerpairs of these materials were used.

Referring back to FIG. 4, when the device is energized, electrons fromsubstrate 410 pass through the intervening layers into emitter layer440. At the same time holes from the anode 470 pass through theintervening layers into emitter layer 440. The electrons and holesrecombine in the emitter layer to produce infrared light. This light isreflected off the two photonic crystal stacks and the photon densitybuilds up in the cavity consisting of layers 430, 440 and 450. This highphoton density produces stimulated emission at a sufficient level thatlasing is induced. Devices like that shown in FIG. 4 have become knownas VCSELs (vertical cavity, surface emitting lasers) and are nowcommonly available.

U.S. Pat. No. 6,160,828 describes an organic Type 1 device known as aVCSEL an example of which, 600, is portrayed in FIG. 6. The deviceconsists of a transparent substrate 610, a first mirror layer 620, afirst electrode 630, a hole transporting layer 640, an emitter layer650, an electron transporting layer 660, a second electrode 670, and asecond mirror layer 680. Layers 640, 650, and 660 are composed oforganic materials. The patent states that the first mirror layer and/orthe second mirror layer are distributed Bragg reflectors. DistributedBragg reflectors are one-dimensional photonic crystal structures builtup by successive deposition of layers of alternating high and lowrefractive index. As was the case in the VCSELs described in U.S. Pat.No. 4,949,350, electrons and holes recombine in the emitter layer 650 ofthis device producing light that is then reflected by the mirror layersback through the emitter layers stimulating further light emission. Ahigh enough photon density is built up in the cavity between the mirrorsto initiate lasing

What both U.S. Pat. Nos. 4,949,350 and 6,160,828 have in common is thatthe photonic crystal structures disclosed as being used on either sideof the cavity that confines light in the laser cavities are formed bysuccessive deposition of many layers of light transmitting material withalternating high and low refractive indices. In the case of OLEDs thefabrication techniques used to fabricate these devices are vacuumevaporation or sputtering. In order to produce efficiently workingdevices, it was necessary to deposit these very thin layers and the verythin layers within the laser micro-cavities with a high degree ofaccuracy in terms of thickness.

FE-OLEDs in which the photonic crystal structures are built up bysuccessive deposition of layers of transparent material have photoniccrystal structures in which the refractive index profile isdiscontinuous. That is to say the refractive index changes abruptly anddiscontinuously at layer boundaries in the photonic crystal structure.This layered structure requires accurate vacuum deposition of manylayers, which becomes impractical if low manufacturing costs arerequired or as devices become larger in size. Therefore, a similardevice having a simpler method of fabrication is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates plots of the density of states versus light frequencyare shown for free space and for a photonic crystal medium.

FIG. 2 illustrates an exemplary structure of an alighted chiral liquidcrystal.

FIG. 3 illustrates an exemplary conventional organic light emittingdiode.

FIG. 4 illustrates an exemplary inorganic Type 1 structure.

FIG. 5 illustrates an exemplary inorganic Type 1 structure.

FIG. 6 illustrates an exemplary VCSEL.

FIG. 7 illustrates various embodiments of a chiral liquid crystallinelight emitting device.

FIG. 8 illustrates a representation of a representative holetransporting, photoalignment layer.

FIG. 9 photopolymerized copolymer of a monomeric hole transporting.

FIG. 10 material monomeric liquid crystal photoalignment material.

FIG. 11 illustrates a representative strongly electron acceptingmonomeric dopant.

FIG. 12 illustrates representative hole transporting liquid crystallinematerials.

FIG. 13 illustrates representative chiral dopants.

FIG. 14 illustrates a representative emitter material.

FIG. 15 illustrates representative emitter host materials.

FIG. 16 illustrates representative dopants and host materials for anemissive layer.

FIG. 17 illustrates representative electron and hole transporting hostmaterials.

FIG. 18 illustrates representative electron transport materials andchiral dopants.

FIG. 19 illustrates a representative n type dopant.

DETAILED DESCRIPTION

In current OLEDs, e.g. those described by FIG. 3, the total thickness ofall the organic layers is less than 200 nanometers and the emitterlayers have thicknesses in the range of 25 to 50 nanometers. Theselayers are thin enough that the low charge carrier mobilities of thematerials do not result in the introduction of large impedances. Bycontrast the chiral liquid crystalline layers doped with fluorescentdyes previously discussed are on the order of 20 microns in thicknessand would have similarly low charge carrier mobilities. Thus, it isexpected that in order to produce a band-edge emitting Type 2 laserdevice, given similar refractive index variation, the emitter layerwould have to contain a photonic crystal structure around 20 microns inthickness with luminescent material doped throughout it. With an emitterlayer that thick in the device very large impedance losses will occur inthe OLEDs causing the devices to run at very elevated voltages andsuffer thermal failure.

The Type 1 photonic crystal structures devices include holographicmirrors (plane wave holograms) and self-assembled structures such aschiral liquid crystals and synthetic opals. Devices with these types ofphotonic crystal structures are described in U.S. patent applicationSer. No. 10/434,941. These three types of photonic crystal structureshave the advantage that they can be produced in a monolithic structure,in one fabrication step, as opposed to the layer by layer approach usedfor the VCSEL devices described above. However, this gives rise to aserious problem. The top photonic crystal structure in such a devicemust be phase registered with the bottom photonic crystal structure orthe desired photon density maximum at the emitter layer will not occur.The potential spatial registration of two holographic reflectors by aninterferometric method has not been successfully implemented in highyields. Thus, until this time there is no proven method to fabricateType 1 devices containing these types of photonic crystal structures inhigh yields at low costs for consumer applications.

A solid state Type 2 device (devices in which light is generated inband-edge modes) differs from the chiral laser devices described inparagraph 0009 in that the light emitting material is located in asingle layer or zone, as opposed to being doped throughout a liquidcrystal. Contrary to what one would expect, even though photons aredistributed throughout the photonic crystal structure, the photondensity in the emitter-containing layer could be made to be sufficientlyhigh so as to yield nearly completely stimulated emission from the lightemitting material in a non-laser device. Thus, as was described in U.S.Provisional Patent Application 62/183,771 it is possible to builddevices in which the organic layers (the hole injection, holetransporting, light emitting, electron transporting and optional chargecarrier blocking layers) are all included in a single layer or zone ofthe photonic crystal structure. In this way, band-edge emitting organiclight emitting diodes (BE-OLEDs) are produced with luminous efficaciesin excess of 300 lumens per watt.

The BE-OLEDs produced in accordance with 62/183,771 are highlyadvantageous in terms of their energy efficiency, operating lifetimes,and the color gamut that they can be made to produce. However, thefabrication of a single monochrome BEOLED device requires the vacuumdeposition of around 20 layers of material. A band-edge emitting OLEDdevice with simpler, more easily manufacturable structure in which thephotonic crystal structure is produced utilizing chiral liquid crystalsis disclosed.

The basic concept behind the invention is that a series ofphotopolymerizable, chiral liquid crystalline layers can be solvent caston a substrate. As each layer is deposited, it is photopolymerized,locking the chiral liquid crystal structure in place in a robust polymermatrix. When the next layer is solvent cast on top of the precedinglayer, the uppermost layer of molecules in the underlying polymerizedlayer acts as a template aligning the bottom layer of molecules in thefluid overlying layer. Once this overlying layer is properly aligned itis in turn photopolymerized. If required, a third and then still morephotopolymerizable chiral liquid crystal layers can be solvent cast ontop of the second, in turn being aligned and photopolymerized.

The mixture of chiral materials in each successive layer may be blendedin such a way that each layer has the same chiral pitch. Further thechiral materials in each layer may also be blended so that the ordinaryand extraordinary refractive indices in each layer match the otherlayers. By matching the chiral pitch and the ordinary and extraordinaryrefractive indices of the materials in this way, the resulting assemblyof layers is an optically uniform assembly of chiral liquid crystallinepolymer. That is to say, the complete assembly of layers will opticallyfunction as a single relatively thick layer of chiral liquid crystal. Animportant aspect of such a layered assembly is that the pitch of thehelical structure of the chiral liquid crystal and its vertical (normalto the layer boundaries) position in space need in no way be related tothe layer boundaries within the structure. If the central layers in thechiral liquid crystal assembly are the active organic layers of an OLED(e.g. the emitter layer, the charge transporting and injection layers),these layers could be located in any vertical position relative to therefractive index alternation and could have any thickness dictated bythe electrical requirements of the device. Thus in the chiral devicestructure the electronic device structure is deconstrained from theoptical device structure.

FIG. 7 portrays various embodiments of a chiral liquid crystalline lightemitting device. The device is built up layer by layer on a transparentsubstrate 71 composed of any suitable materials, for instance glass orclear plastic. First a clear, electrically conducting anode layer 72 iscoated onto the substrate, most often by vacuum deposition. This anodelayer may comprise a transparent semiconducting material such as indiumzinc oxide or indium-tin oxide. Next a hole transporting, liquid crystalphotoalignment layer 73 is coated onto the substrate surface. U.S. Pat.No. 7,118,787 and US Patent Application 2011/0020566 disclose variousmaterials that can be used as such a hole transporting layer in variousembodiments of a chiral liquid crystalline light emitting device. Forexample, an exemplary prepolymerized and solvent cast hole transportingliquid crystal photoalignment material 800, is shown in FIG. 8.Alternatively an exemplary photopolymerized copolymer of a monomerichole transporting material 900 (illustrated in FIG. 9) and a monomericliquid crystal photoalignment material 1000 (illustrated in FIG. 10) maybe used to form layer 73. In this case, layers are produced by solventcasting a mixture of the two monomers onto the surface of the anodelayer and photopolymerizing them in place. Liquid crystal photoalignmentlayers of this type are produced by first forming the layer as describedabove and then exposing it to polarized UV light (e.g. at a wavelengthof 325 nm). This polymerizes the double bonds in the coumarin functionalunits aligned along one axis in the in the molecules of the layer, butnot along the perpendicular axis in the plane of the layer. This, inturn, produces an anisotropic surface energy at the surface of thealignment layer that aligns liquid crystal molecules coated over it.

The next layer to be fabricated as shown in FIG. 7 is a p-doped holetransporting layer 74. The goal in the fabrication of this layer is toproduce a relatively high conductivity film that minimizes the voltagedrop between the anode and the emitter layer 76. This is accomplished bydoping a monomeric chiral liquid crystalline hole transporting materialwith a strongly electron accepting monomeric p-dopant 1100 illustratedin FIG. 11.

In order to achieve a complete photonic stop band in a relatively thinlayer of chiral material it is necessary that the chiral nematicmonomers used to produce layer 74, shown in FIG. 7, and the other chiralliquid crystalline layers in the device have a very high ratio of theirextraordinary refractive indices to their ordinary refractive indices.This, in turn, means that their molecules' long axes be much longer thanthe molecules are wide. An example of such a hole transporting liquidcrystalline material 1200 is illustrated in FIG. 12. An alternaterepresentation of such a hole transporting liquid crystalline material1210 is also shown, where n=3 and m=10. Materials with m having othervalues between 5 and 12 may also be used. Another example of these typesof materials 1220 is also shown in FIG. 12. Here material 1220, has an nvalue of 3 and an m value of between 5 and 12, may be used in thislayer.

Since all of the chiral liquid crystalline polymer layers in the deviceneed to be matched in terms of ordinary and extraordinary refractiveindices, it may be necessary to introduce less birefringent liquidcrystalline monomer materials into the mixture of materials in thislayer to tune the refractive indices. For instance, compounds having theabove formulas with n=1 or 2 might be used as additives to tunerefractive index.

In order to produce a chiral liquid crystalline polymer of the requiredpitch, it is necessary to dope the monomer mixture used to produce layer74 in FIG. 7 with a chiral dopant. Exemplary chiral dopants areillustrated by general formula 1300 shown in FIG. 13. Chiral dopants1300 have an n value of 1 to 3 and an m value of 5 to 12 and themolecular center of asymetry is highlighted by the asterisk.Alternatively chiral dopants illustrated by formula 1310, also shown inFIG. 13 may be used, dopants 1310 have an n value of 1 to 3 and an mvalue of 5 to 12 and the molecular center of asymmetry is highlighted bythe asterisk.

Referring back to FIG. 7 the mixture of monomers and p-dopant 1100 to beused to form layer 74 is solvent cast over the photoalignment layer 73and solvent is allowed to evaporate off either at room temperature or anelevated temperature. Once the solvent is gone the material is in theform of a layer of chiral nematic liquid crystalline fluid or possibly achiral nematic liquid crystalline glass. The anisotropic surface energyat the top surface of layer 73 induces the molecules of the chiralnematic material at the interface between layers 73 and 74 to be alignedwith their molecular long axes surface parallel and with their molecularlong axes all substantially in the same direction thus producing thedesired helical structure in layer 74. [0050] The next layer to befabricated in the layer by layer process is the hole transporting layer75. The function of this layer is to convey electrically conductingholes from p-doped layer 74 into emitter layer 76. In doing so the holestransition in energy from the highest occupied molecular orbital (HOMO)levels in the conductive dopant in layer 74 to the HOMO energy levels ofemitter layer 76. The layer is produced by the solvent casting of asolution of a mixture of chiral nematic liquid crystalline monmers in amanner similar to that used to produce layer 74 except that there is nop-dopant in the mixture. After the solvent used to cast the monomericmixture evaporates away the material of the mixture forms a layer ofchiral nematic liquid crystalline fluid or a chiral nematic glass on thetop surface of layer 74. The liquid crystalline order of layer 74provides a template such that the long axes of the molecules at thebottom surface of layer 75 are aligned parallel with the long axes ofthe nematic molecular cores of the polymeric material at the top surfaceof layer 74. In this way the helical structure induced by the chiralnature of the materials in layers 74 and 75 is continuous in passingacross the interface between the two layers. Once the material of layer75 is in place and properly aligned it is polymerized by exposure toultraviolet light. The chiral nematic monomeric materials that are usedto form layer 75 may be the same as were used to produce layer 74 exceptthat the p-dopant is omitted. The exact chemical structure of themolecules used and their molecular lengths is dictated by therequirement that the ordinary and extraordinary refractive indexcomponents of the mixtures in the two layers must match each other andthe helical pitches of the chiral nematic structures within the twolayers must also be the same.

The emitter layer 76 of the device may consist solely of a monomericnematic electroluminescent material doped with a chiral additive toproduce a helical structure of the proper pitch. One suitableelectroluminescent nematic material 1400 is illustrated in FIG. 14.However, a more preferable formulation for the material in this layer isto utilize an electroluminescent dopant doped into a host composed of amixtutre of monomeric chiral nematic materials. This approach has anumber of advantages, for example, the dopant concentration may bechosen so as to minimize self-absorption of light by the dopant; themonomeric host chiral nematic materials may be blended to produce amixture that is ambipolar, that is to say, electron and hole mobilitiesare approximately equal; and non-liquid crystalline emitter materialssuch as phosphorescent emitters with very high quantum efficiencies maybe utilized as dopants while maintaining the desired helical structureof the chiral host.

Rays of light that are entrained in the helical photonic crystalstructure have their associated electric vectors all oriented parallelto the planes of the device layers. As a result, this light will onlyinteract with excited molecules whose transition moments are alsosubstantially in the plane of the device. Therefore, electroluminescentmaterials whose molecules are preferentially oriented so that theirtransition moments are in the device plane will yield the highest deviceenergy efficiencies (external quantum efficiencies) when used in theemitter layers of the devices of this invention.

Some emitter layer host materials spontaneously align with their longaxes in the plane of the emitter layer upon vacuum deposition in thefabrication of small molecule OLEDs. When these host materials are dopedwith phosphorescent heteroleptic phosphorescent iridium organometallicdopants containing general structure 1500, illustrated in FIG. 15, thelight emitting dopants are aligned with their transition moments in theplane of the layer.

In these molecules the iridium is complexed to a single molecule of thebidentate ligand acetylacetone (2,4-cyclopentanedione). It is also boundby covalent iridium to carbon bonds to two bidentate ligands each ofwhich are also bound to the iridium by a coordinate iridium to nitrogenbond. An example of such a molecule, 1510, is Ir(ppy)₂(acac).

When doped at a concentration of 8% into the host material4,4′-bis(carbazol-9yl)biphenyl (CBP) in the emissive layer of a vacuumdeposited, small molecule OLED device, this green light emissivematerial yields unusually high emission efficiency because the moleculesof the phosphorescent material are aligned with their transition momentslargely inplane. A monomeric version, 1520 of this material may be dopedinto a chiral nematic monomer host and the resulting mixture is used toform layer 76 of FIG. 7.

Similarly, the red emissive material, 1600 illustrated in FIG. 16,bis(2methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium III[Ir(MDQ)₂(acac)] may be doped into the host materialN,N′di(naphthalen-1-yl)-N,N′-diphenylbenzidene (NPB) in the emissivelayer of a vacuum deposited, small molecule OLED device yieldingin-plane alignment of the emitter transition moments A monomericversion, 1610 of this material is also illustrated in FIG. 16, which maybe doped into a chiral nematic monomer host and this mixture used toform layer 76.

The use of an ambipolar host material in layer 76 allows for balancedinjection of electrons and holes into the emitter layer withrecombination to yield excitons occurring away from the layer boundariesavoiding traps that may exist at the layer boundaries. Blending a holetransporting, monomeric, chiral nematic host material with an electrontransporting, monomeric, chiral nematic host material may be used toproduce an ambipolar host material. Examples of hole transporting hostmaterials are illustrated by general formula 1700 in FIG. 17.

Similarly examples of electron transporting host materials areillustrated by general formula 1710. These materials that may be blendedwith materials 1700 to produce an ambipolar host mixture are also shownin FIG. 17.

By blending appropriate amounts of the two type of compounds, anambipolar mixture can be achieved. Further, by blending materials withsuitable values of n, a mixture with the right extraordinary andordinary refractive indices to match the indices of layers 74 and 75,the helical pitch of the mixture may be adjusted based on the amount ofa chiral dopant added. Examples of chiral dopants, 1720 are illustratedin FIG. 17, where m=5 to 12 and the molecular center of assymetry ishighlighted by the asterisk.

The next device layer to be fabricated is the electron transportinglayer 77. The function of this layer is to convey electrons from n-dopedlayer 78 into emitter layer 76. In doing so the electrons transition inenergy from the electron energy levels in the conductive dopant in layer78 to the HOMO energy levels of emitter layer 76. The layer is producedby the solvent casting of a solution of a mixture of chiral nematicliquid crystalline monmers in a manner similar to that used to produceprevious layers. After the solvent used to cast the monomeric mixtureevaporates away, the material of the mixture forms an aligned layer ofchiral nematic liquid crystalline fluid or a chiral nematic glass due tothe template effect from the underlaying layer. In this way the helicalstructure induced by the chiral nature of the materials in layer 77 andall the previous layers in the device is continuous in passing acrossthe interfaces between the two layers. The helical structure ispolymerized so as to lock it into place by exposure to UV light.

Electron transporting 77 is fabricated from a mixture of monomericnematic materials, examples 1800 of which are illustrated in FIG. 18where n is 1 to 3 and m is 5 to 12. By blending materials that aremembers of this series with suitable values of n, a mixture with theright extraordinary and ordinary refractive indices to match the indicesof layers 74, 75 and 76 may be formulated. The helical pitch of themixture may be adjusted based on the amount of a chiral dopant added.Examples of chiral dopants 1810 that may be used are illustrated in FIG.18 where n=1 to 3, m=5 to 12, and the molecular center of assymetry ishighlighted by the asterisk. Examples of other chiral dopants, 1820,that may be used are further illustrated in FIG. 18, where m=5 to 12 andthe molecular center of assymetry is highlighted by the asterisk.

The next device layer to be fabricated is the n-doped electrontransporting layer 77. The function of this layer is to convey electronsfrom the cathode 710 or possibly from an optional electron injectionlayer 78 into the electron transporting layer 77. In doing so theelectrons transition in energy from the electron energy level of thecathode work function to the lowest unoccupied molecular orbital (LUMO)energy levels of the electron transporting layer 77. The layer isproduced by the solvent casting of a solution of a mixture of chiralnematic liquid crystalline monmers in a manner similar to that used toproduce previous layers. The layer also incorporates an n-dopant, 1900such as the dopant(4-(1,3-dimethyl-2,3-dihydro-1Hbenzoimidazol-2-yl)phenyl)dimethylamine(N-DBMI), as illustrated in FIG. 19, which is activated by heating afterthe film is solvent cast.

After the solvent used to cast the monomeric mixture evaporates away,the material of the mixture forms an aligned layer of chiral nematicliquid crystalline fluid or a chiral nematic glass due to the templateeffect from the underlaying layer. In this way the helical structure 79induced by the chiral nature of the materials in layer 78 and all theprevious layers in the device is continous in passing across theinterfaces between the two layers. The helical structure is polymerizedso as to lock it into place by exposure to UV light.

Layer 78 may fabricated using the same mixture of monomeric nematicmaterials as were used for layer 77, for example 1800, where n=1 to 3and m=5 to 12. By blending materials that are members of this serieswith suitable values of n a mixture with the right extraordinary andordinary refractive indices to match the indices of layers 74, 75, 76,and 76 may be formulated. The same chiral dopants may also be used toadjust the pitch of the helical structure, for example 1810 where n=1 to3, m=5 to 12, and the molecular center of assymetry is highlighted bythe asterisk as shown in FIG. 18; and 1820 where m=5 to 12 and themolecular center of assymetry is highlighted by the asterisk as alsoshown in FIG. 18.

Optionally layer 78 may be capped with an electron injection layer 710.Layer 710, for example, may comprise lithium fluoride or cesiumcarbonate. Layer 711 is a cathode and may, for example, be made from alow work function metal such as aluminum. A major advantage of thedevices of this invention over those of U.S. Provisional Application62/183,771 is that cathode 711 is not integral to the photonic crystalstructure and therefore need not be transparent or have a particularlywell controlled thickness. The cathodes in the devices of U.S.Provisional Application 62/183,771 must be transparent as possible toavoid light loss and constitute a refractive index zone or part of azone in the photonic crystal and thus must have very tightly controlledthicknesses. To meet these requirements the cathodes in the devices ofU.S. Provisional Application 62/183,771 are often multilayered in natureand are challenging to fabricate properly. The cathodes of the devicesin this invention are simple and easily fabricated.

When device 7100 is energized, holes flow from anode 72 through holeinjection layer 72 and layers 74 and 75 into emitter layer 76. At thesame time electrons flow from cathode layer 711 through layers 710, 78and 77 into emitter layer 76. The electrons and holes recombine onluminescent material molecules in layer 76 yielding excitons. Sinceemitter layer 76 is inside a photonic crystal structure, excitonscreated in that layer cannot emit light at wavelengths in the stop-bandof the photonic crystal. However, where the emission band of theluminescent material in layer 76 overlaps the band-edge wavelengths ofthe stop-band, light emission does occur and because of the high densityof states at those wavelengths unusually high levels of emission occur.The photonic crystal traps the light from band-edge emission within itsstructure increasing the photon density to the point where there aresufficient photons to interact with excitons to the extent that nearlyall light emission is stimulated emission. There is, however, aninsufficient level of stimulated emission to produce lasing. Since thelight from stimulated emission is almost completely vertical in itsdirection of propagation within the device, there is very little lossdue to internal reflection and trapping of light and the device is as aresult highly energy efficient.

What is claimed is:
 1. A light emitting device, comprising: a first holetransporting layer comprising a polymerized chiral nematic liquidcrystal material that is doped with a p-dopant; a second holetransporting layer comprising a polymerized chiral nematic liquidcrystal material; a light emitting layer comprising a polymerized chiralnematic liquid crystalline material; an electron transporting layercomprising a polymerized chiral nematic liquid crystal material; whereinsaid polymerized chiral nematic liquid crystal materials in each layerhave an extraordinary refractive index and an ordinary refractive indexand a difference between said extraordinary and ordinary refractiveindices of the polymerized chiral nematic liquid crystal material ineach of said layers varies by less than 10% from a difference betweensaid extraordinary and ordinary refractive indices of the polymerizedchiral nematic liquid crystal material in each other of said layers. 2.The light emitting device of claim 1 wherein the extraordinaryrefractive indices of the polymerized chiral nematic liquid crystalmaterial in each of said layers vary by less than 10% from theextraordinary refractive indices of the polymerized chiral nematicliquid crystal material in each of the other said layers.
 3. The lightemitting device of claim 1 wherein the ordinary refractive indices ofthe polymerized chiral nematic liquid crystal material in each of saidlayers vary by less than 10% from the ordinary refractive indices of thepolymerized chiral nematic liquid crystal material in each of the othersaid layers.
 4. The light emitting device of claim 1 further comprising:an anode; and a hole injection layer comprising a liquid crystalalignment layer; wherein said hole injection layer lies between saidanode and said first hole transporting layer.
 5. The light emittingdevice of claim 4 wherein said liquid crystal alignment layer comprisesliquid crystal alignment properties that are induced by exposure toplane polarized ultraviolet light.
 6. The light emitting device of claim1 wherein each of said layers comprises a plane and the chiral nematicliquid crystal material comprising each one of said layers comprisesmolecules with long axes aligned parallel with said plane of each one ofsaid layers.
 7. The light emitting device of claim 1 wherein the chiralnematic material in each one of said layers spontaneously adopts ahelical structure with a helical pitch, wherein said helical pitch ofsaid helical structure of the chiral nematic material in each of saidlayers is the same within each layer.
 8. The light emitting device ofclaim 1 wherein the chiral nematic material comprising the lightemitting layer is doped with an electroluminescent material.
 9. Thelight emitting device of claim 8 wherein said light emitting layerfurther comprises a plane and said electroluminescent material comprisesmolecules with transition moments responsible for electroluminescentlight emission aligned parallel with said plane of the layer the lightemitting layer.
 10. The light emitting device of claim 1 wherein each ofsaid layers has a thickness and said light emitting layer has athickness that is 10% or less of the a total thickness that is a sum ofeach respective thickness of all the layers each respective layercomprising polymerized chiral nematic liquid crystal material combined.11. A light emitting device, comprising: a hole transporting layercomprising a polymerized chiral nematic liquid crystal material; a lightemitting layer comprising a polymerized chiral nematic liquidcrystalline material; a first electron transporting layer comprising apolymerized chiral nematic liquid crystal material; and a secondelectron transporting layer comprising a polymerized chiral nematicliquid crystal material that is doped with an n-dopant, wherein saidpolymerized chiral nematic liquid crystal materials in each layer havean extraordinary refractive index and an ordinary refractive index and adifference between said extraordinary and ordinary refractive indices ofthe polymerized chiral nematic liquid crystal material in each of saidlayers varies by less than 10% from a difference between saidextraordinary and ordinary refractive indices of the polymerized chiralnematic liquid crystal material in each other of said layers.
 12. Thelight emitting device of claim 11 wherein the extraordinary refractiveindices of the polymerized chiral nematic liquid crystal material ineach of said layers vary by less than 10% from the extraordinaryrefractive indices of the polymerized chiral nematic liquid crystalmaterial in each of the other said layers.
 13. The light emitting deviceof claim 11 wherein the ordinary refractive indices of the polymerizedchiral nematic liquid crystal material in each of said layers vary byless than 10% from the ordinary refractive indices of the polymerizedchiral nematic liquid crystal material in each of the other said layers.14. The light emitting device of claim 11 further comprising: an anode;and a hole injection layer comprising a liquid crystal alignment layer;wherein said hole injection layer lies between said anode and said firsthole transporting layer.
 15. The light emitting device of claim 14wherein said liquid crystal alignment layer comprises liquid crystalalignment properties that are induced by exposure to plane polarizedultraviolet light.
 16. The light emitting device of claim 11 whereineach of said layers comprises a plane and the chiral nematic liquidcrystal material comprising each one of said layers comprises moleculeswith long axes aligned parallel with said plane of each one of saidlayers.
 17. The light emitting device of claim 11 wherein the chiralnematic material in each one of said layers spontaneously adopts ahelical structure with a helical pitch, wherein said helical pitch ofsaid helical structure of the chiral nematic material in each of saidlayers is the same within each layer.
 18. The light emitting device ofclaim 11 wherein the chiral nematic material comprising the lightemitting layer is doped with an electroluminescent material.
 19. Thelight emitting device of claim 18 wherein said light emitting layerfurther comprises a plane and said electroluminescent material comprisesmolecules with transition moments responsible for electroluminescentlight emission aligned parallel with said plane of the layer the lightemitting layer.
 20. The light emitting device of claim 11 wherein eachof said layers has a thickness and said light emitting layer has athickness that is 10% or less of the a total thickness that is a sum ofeach respective thickness of all the layers each respective layercomprising polymerized chiral nematic liquid crystal material combined.